14.7. Error: JS Modules not supported

This is most likely because you are using a very old browser, or have some Javascript disabled.

This page describes the compatible browsers for Javascript modules; https://caniuse.com/es6-module .

There is no workaround for this apart from updating your browser / using a better browser, as OliveTin wants to be a progressive web app and doesn’t intend to support browsers without module support.